Soil Building with Cover Crops

The hottest summer months in Kihei are tough on most garden plants, but it’s a great time to introduce cover crops for improving soil health and preparing for the next planting season. These crops also serve as great tools to teach students about nitrogen and its role in the life cycle.

All plants need nitrogen to make amino acids, proteins and DNA. Approximately 80 percent of the air surrounding the earth is nitrogen gas. However, nitrogen in gas form is not usable by most plants. First, nitrogen needs to be converted from its atmospheric gas form to ammonium compounds to become available to plants. To achieve this, many organic farmers use nitrogen-fixing cover crops. Cover crop benefits and tips include:

Boosting Soil Fertility & Nitrogen Fixing

Cover crops, also known as green manures, recycle nutrients and add organic matter to the soil.  The nutrients are absorbed and stored inside plants. When nutrients are needed for the next crop, the old plants are dug into the soil or used as mulch on top of the soil. We always explain to students, “old plants make food for new plants.”

Legumes – such as alfalfa, peas, beans, clover and vetch – are particularly beneficial to replenish nitrogen available to plants. During the process, called “nitrogen fixing,” rhizobial bacteria take residence in root nodules of legumes and convert biologically unavailable atmospheric nitrogen gas to a plant-available ammonium compound.

Inoculating Seeds for Better Nitrogen Fixing

There are multiple strains of nitrogen-fixing rhizobia, and each is specific to a certain type of legume. For instance, the rhizobia strain associated with vetch will also work with peas, but not alfalfa. When legumes are inoculated with the proper strain of rhizobial bacteria, they produce large, pink nodules on the roots of the host plant. The pinkish color indicates the presence of a hemoglobin-like molecule that is necessary for nitrogen fixation to occur.

Rhizobia live naturally in the soil. However, the strains already present may not always be compatible for your cover crop or in the amount necessary for effective nitrogen fixation. To increase the odds, many gardeners inoculate their cover crop seeds by lightly dusting them with the appropriate rhizobial bacteria strains prior to planting. Rhizobial bacteria powder may be found in or near the seed section at your local nursery. Check the label to ensure the inoculant matches your cover crop seed of choice. We’ve had difficulty finding inoculant on store shelves on Maui, so ordering online is the best option here. Peaceful Valley has a great selection of seed inoculants with instructions on proper use of each strain.

Improving Soil Structure & Habitat

Cover crops strengthen soil structure, letting more air into the soil, improving drainage, maintaining viable living space for beneficial microorganisms and insects. Cover crops can also help the sandy soils found in Kihei hold more water. While some cover crops are drought resistant, beneficial bacteria generally fare better with moisture.

Preventing Soil Erosion and Compaction

Cover crops help prevent soil from being carried away by wind and rain. The roots penetrate the soil and hold it in place. Having plants constantly growing in your gardening spaces also gives a visual cue to help deter kids from marching through and compacting the soil.

Controlling Weeds

Bare soil can become quickly overgrown with weeds, which can be difficult to remove once they’ve become established. A good ground cover can prevent weeds from growing by competing for nutrients, space and light.

Want to learn more? You’re invited to help us plant new cover crops and learn hands-on how to improve nitrogen availability for healthy new garden plants. Attend our upcoming Work & Learn Days or email info@GrowSomeGood.org for more information on participating in classes with Kihei Elementary & Lokelani Intermediate students.

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